linux-kernel - Re: [PATCH v6 2/9] x86, kfence: enable KFENCE for x86

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Message-ID: <CANpmjNNBoiL2=JDD=vC5dB_TPW1Ybe5k7SqqhvUE2B7GmzRLyg@mail.gmail.com>
Date:   Fri, 30 Oct 2020 14:00:09 +0100
From:   Marco Elver <elver@...gle.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hillf Danton <hdanton@...a.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Jonathan Corbet <corbet@....net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Jörn Engel <joern@...estorage.com>,
        Kees Cook <keescook@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        SeongJae Park <sjpark@...zon.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v6 2/9] x86, kfence: enable KFENCE for x86

On Fri, 30 Oct 2020 at 03:49, Jann Horn <jannh@...gle.com> wrote:
> On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@...gle.com> wrote:
> > Add architecture specific implementation details for KFENCE and enable
> > KFENCE for the x86 architecture. In particular, this implements the
> > required interface in <asm/kfence.h> for setting up the pool and
> > providing helper functions for protecting and unprotecting pages.
> >
> > For x86, we need to ensure that the pool uses 4K pages, which is done
> > using the set_memory_4k() helper function.
> >
> > Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>
> > Co-developed-by: Marco Elver <elver@...gle.com>
> > Signed-off-by: Marco Elver <elver@...gle.com>
> > Signed-off-by: Alexander Potapenko <glider@...gle.com>
> [...]
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> [...]
> > @@ -725,6 +726,9 @@ no_context(struct pt_regs *regs, unsigned long error_code,
> >         if (IS_ENABLED(CONFIG_EFI))
> >                 efi_recover_from_page_fault(address);
> >
> > +       if (kfence_handle_page_fault(address))
> > +               return;
>
> We can also get to this point due to an attempt to execute a data
> page. That's very unlikely (given that the same thing would also crash
> if you tried to do it with normal heap memory, and KFENCE allocations
> are extremely rare); but we might want to try to avoid handling such
> faults as KFENCE faults, since KFENCE will assume that it has resolved
> the fault and retry execution of the faulting instruction. Once kernel
> protection keys are introduced, those might cause the same kind of
> trouble.
>
> So we might want to gate this on a check like "if ((error_code &
> X86_PF_PROT) == 0)" (meaning "only handle the fault if the fault was
> caused by no page being present", see enum x86_pf_error_code).

Good point. Will fix in v7.

> Unrelated sidenote: Since we're hooking after exception fixup
> handling, the debug-only KFENCE_STRESS_TEST_FAULTS can probably still
> cause some behavioral differences through spurious faults in places
> like copy_user_enhanced_fast_string (where the exception table entries
> are used even if the *kernel* pointer, not the user pointer, causes a
> fault). But since KFENCE_STRESS_TEST_FAULTS is exclusively for KFENCE
> development, the difference might not matter. And ordering them the
> other way around definitely isn't possible, because the kernel relies
> on being able to fixup OOB reads. So there probably isn't really
> anything we can do better here; it's just something to keep in mind.
> Maybe you can add a little warning to the help text for that Kconfig
> entry that warns people about this?

Thanks for pointing it out, but that option really is *only* to stress
kfence with concurrent allocations/frees/page faults. If anybody
enables this option for anything other than testing kfence, it's their
own fault. ;-)
I'll try to add a generic note to the Kconfig entry, but what you
mention here seems quite x86-specific.

Thanks,
-- Marco